1. Field of the Invention
This invention relates to pipes that convey oil, such as oil well pipes and oil pipelines, and more particularly to coatings for the interior surface of such pipes.
2. Description of Related Art
Oil pipes for conveying large volumes of oil have two primary utilities, as down-hole pipes for conveying oil from underground deposits to the earth surface and as pipelines for the long distance transportation of oil across the earth surface. Such pipes are large and long, usually having an inner diameter of at least 2 in (5.08 cm) and length of at least 10 ft (3 m), more often at least 20 ft (6.1 m) and often a length of at least 30 ft (9.1 m). Such pipes are typically made from carbon steel for economy reasons, rather than expensive specialty metal alloys that better resist the corrosive entities in the crude oil. The corrosion is especially severe in the hot underground environment of the oil deposit from such materials as water, sulfur, sulfur dioxide, carbon dioxide, hydrogen sulfide, present in the oil typically making it acidic. These materials corrode the oil pipe even at relatively low temperatures of oil transportation; the long contact times with the oil pipeline interior surface provide the conditions for corrosion to occur. An additional problem arises from soluble organic material present in the oil at the high temperature of the oil deposit, such as asphaltenes and paraffin waxes and with soluble inorganic material, commonly referred to as scale and generally comprising calcite and/or barite, present in the oil or in the presence of salt water associated with the conveying of oil from underground deposits. These materials become insoluble as the oil cools, as occurs during the rise of the oil through the down-hole pipe to the earth surface. The resultant insoluble materials tend to plate out on the interior surface of the pipe, restricting the oil flow therethrough and eventually plugging the pipe. This also occurs during long distance conveying of the oil through pipelines. This requires the oil pipes to be cleaned out, during which time oil production or transportation, as the case may be, ceases.
It is known to line the interior surface of oil well pipes with a fluoropolymer, such as polytetrafluoroethylene (PTFE), for example, as disclosed in EP 0 1910 092 to Mannesman Akt. Such fluoropolymer linings present a non-stick surface to the oil. However, because of this non-stick property, these linings do not adhere to the interior surface of the pipe.
Thus, there remains a need for solving the problems of corrosion and pluggage occurring in oil conveying pipes.